1. Technical Field
The present invention relates to a differential gear for a motor vehicle, having a powered drive cage, two driven shafts, connected with the drive cage via a planetary or differential gear, as well as a stationary gear housing. An intermediate gear with a planetary unit is provided per driven shaft, the planetary unit includes a plurality of planetary wheel pairs and a planetary wheel support, the planetary wheel pairs are arranged on planetary wheel pair shafts, which are connected with the planetary wheel support and each has a first planetary wheel and a second planetary wheel. The first planetary wheel and the second planetary wheel are arranged coaxially with respect to each other and are connected with each other in a manner fixed against relative rotation and the first planetary wheel and the second planetary wheel have different effective diameters. In addition, the first planetary wheel is in engagement with a first sun wheel, which is connected with the drive cage in a manner fixed against relative rotation, and the second planetary wheel is in engagement with a second sun wheel, which is connected with the driven shaft in a manner fixed against relative rotation and arranged coaxially with respect to the first sun wheel. A brake device is provided, which has a breaking force applicator which causes a breaking action, and the braking torque is capable of braking the planetary unit with respect to the stationary gear housing, and wherein the planetary unit has active surfaces provided for introducing the breaking torque into the planetary unit.
2. Background Information
Differential gears are used for the transverse compensation of the different number of revolutions of the wheels which results from the different lengths of the tracks when driving around curves. In the course of this, it is intended as a rule to transfer the driving action symmetrically, and therefore free of yawing action, to the individual wheels. However, it can be desirable in certain driving situations, or for affecting the driving characteristics of a motor vehicle, to interfere in a definite manner with the driving power distribution to the individual wheels, and in this way to optimize the driving behavior, or to aid the driver in critical driving situations. This can be provided by brake engagement, as well as by affecting the distribution of the propulsive power from a differential gear to the individual wheels.
A differential gear of the type mentioned at the outset is distinguished in that the planetary unit, which normally rotates along with the drive cage and driven shaft, can be braked or blocked when the brake device is operated.
With the brake device not operated, the planetary unit rotates as a block around the driven shaft. The differential gear performs the functions of a conventional differential. When the brake device is activated, and since the individual planetary wheels of the planetary wheel pairs have differently effective diameters, a torque is transmitted via the intermediate gear to the driven shaft as a result of the braking of the planetary unit which, in comparison with the operation when the brake device is not activated, accelerates or decelerates the driven shaft. A yawing action is intentionally created by this, which is generated on both sides of the vehicle because of the different number of revolutions of the drive wheels, or because of the different drive wheel moments, or number of revolutions of the drive wheels resulting therefrom. In contrast to differentials which merely have an active blocking device which, at full blocking power, permits a maximal synchronization of the number of revolutions of the driven shafts, it can be achieved that even the drive wheel at the outside of the curve which, in conventional differentials, rotates faster than the drive wheel on the inside of the curve because of the longer outer track, is accelerates past the number of wheel revolutions resulting from the track length.
By a defined addition of a yawing action it is possible to interfere in a corrective manner with the driving behavior of the vehicle in various driving situations, or it is possible to create a vehicle with special driving properties, in particular with special curve-steering behavior. Such gears are also called “active yaw” gears because of the option of introducing a yawing action.
Such a differential gear is known from U.S. Pat. No. 6,120,407, the entire contents of which are incorporated herein by reference. In this differential gear, the planetary unit can be braked via a braking device arranged on an outer shell face of a disk-shaped base body of the planetary support. For this purpose the braking device has a stack of friction disks, which can be compressed by a hydraulic unit. In this case active surfaces in the form of interior disks are provided on the planetary support and are arranged, fixed against relative rotation, on the planetary support acting as the inner disk support, while the outer disks are arranged, fixed against relative rotation, on the stationary gear housing.
However, this embodiment has the disadvantage that the braking device and the active surfaces are spaced far apart from the planetary unit, which results in a planetary unit which is structurally elaborate, large, and therefore also comparatively heavy and expensive. The mass of the planetary unit which is to be accelerated and braked is relatively large. Such an embodiment also requires an increased demand for structural space for the entire differential gear. Added to this is that, because of the differing diameters of the first and second planetary wheel, the forces acting on them impart a torque to the planetary unit which, in the known embodiment, must be absorbed by an elaborate seating of the planetary unit. Furthermore, an elaborate axial seating of the planetary unit is also required for the positionally correct seating of the braking device.